The general objective of this research is to determine the effects of centrally acting drugs administered during embryogenesis and the neonatal period on the behavioral development of the young organism. Specifically, the research objective of this proposal is to determine, by pharmacological intervention, the involvement of central nervous system (CNS) transmitters in the development of learned inhibitory stimulus control in the young organism. The development of learned inhibitory stimulus control will be determined by studying the performance of chicks of different ages on behavioral paradigms used to determine inhibitory processes in the adult (e.g., basic orthogonal procedure, combined-cue test, and resistance to reinforcement test). Heat reinforcement techniques now permit these discrimination testing procedures to be used with the young chick. Pharmacological agents will be used to alter synthesis, storage, release or metabolism of putative CNS transmitters before, during, and after discrimination testing. Control procedures will determine whether significant age or drug differences in inhibitory performance are due to changes in sensory, motor, or motivational variables.